Focus on the genitive singular, because that always clues you in to how the noun’s declension actually works. ἅρπαγ-ος > *ἅρπαγ-ς > ἅρπαξ.

Don’t focus on each distinct pattern of the third declension. For passive recognition, it isn’t worth it.

Resign yourself to the fact that the nominative singular will be strange.

Treat the dirty vowel stems (3, 4, 5 in your list) as separate declensions. You can, with enough concentration, discern the recurring endings there too; but you might as well not; there’s been too much intervening sound change to make it worthwhile.

Do not even bother learning the highly irregular patterns, like ναῦς and ἀνήρ and γυνή. They’ll be frequent enough that you’ll pick them up from context anyway.